tomcat集群时统计session与在线人数

　　tomcat集群时，原来通过HttpSessionListener实现类监听session的创建和销毁来统计在线人数的方法不再有效，因为不是每个人登陆都会在同一个tomcat服务器上，而在另一台tomcat上登陆的人的session是通过session复制创建的，而复制过程不会调用HttpSessionListener接口的方法，也一直没找着如何监听session复制的方法，所以就没法统计在线人了。

今天突然回想起tomcat下的manager应用上面就能看到session数和session的内容，于是本文的实现原理就是，做一个类似这样的servlet，此servlet把tomcat上负责管理应用的对象保存下来，供我任意使用。在tomcat上看应用的信息时，使用的是http://localhost:8080/manager/html/list这个路径，页面信息见下图：

　　于是把源码下来看看 (apache-tomcat-6.0.39-src)，细看tomcat下webapps/manager/WEB-INF/web.xml文件配置，发现原来tomcat是通过org.apache.catalina.manager.ManagerServlet这个类来提供以上服务的，跟踪此类doGet方法代码

 // --------------------------------------------------------- Public Methods

     /**
      * Process a GET request for the specified resource.
      *
      * @param request
      *            The servlet request we are processing
      * @param response
      *            The servlet response we are creating
      *
      * @exception IOException
      *                if an input/output error occurs
      * @exception ServletException
      *                if a servlet-specified error occurs
      */
     public void doGet(HttpServletRequest request, HttpServletResponse response)
             throws IOException, ServletException {

         // Identify the request parameters that we need
         // 取得访问路径，
         String command = request.getPathInfo();

         String path = request.getParameter("path");
         String deployPath = request.getParameter("deployPath");
         String deployConfig = request.getParameter("deployConfig");
         String deployWar = request.getParameter("deployWar");

         // Prepare our output writer to generate the response message
         response.setContentType("text/html; charset=" + Constants.CHARSET);

         String message = "";
         // Process the requested command
         if (command == null || command.equals("/")) {
         } else if (command.equals("/deploy")) {
             message = deployInternal(deployConfig, deployPath, deployWar);
             // 找到了，就是这个路径，往下看list方法
         } else if (command.equals("/list")) {
         } else if (command.equals("/reload")) {
             message = reload(path);
         } else if (command.equals("/undeploy")) {
             message = undeploy(path);
         } else if (command.equals("/expire")) {
             message = expireSessions(path, request);
         } else if (command.equals("/sessions")) {
             try {
                 doSessions(path, request, response);
                 return;
             } catch (Exception e) {
                 log("HTMLManagerServlet.sessions[" + path + "]", e);
                 message = sm
                         .getString("managerServlet.exception", e.toString());
             }
         } else if (command.equals("/start")) {
             message = start(path);
         } else if (command.equals("/stop")) {
             message = stop(path);
         } else if (command.equals("/findleaks")) {
             message = findleaks();
         } else {
             message = sm.getString("managerServlet.unknownCommand", command);
         }
         // 就是这个方法生成上面的那个页面
         list(request, response, message);
     }

list

 /**
      * Render a HTML list of the currently active Contexts in our virtual host,
      * and memory and server status information.
      *
      * @param request
      *            The request
      * @param response
      *            The response
      * @param message
      *            a message to display
      */
     public void list(HttpServletRequest request, HttpServletResponse response,
             String message) throws IOException {

         if (debug >= 1)
             log("list: Listing contexts for virtual host '" + host.getName()
                     + "'");

         PrintWriter writer = response.getWriter();

         // HTML Header Section
         writer.print(Constants.HTML_HEADER_SECTION);

         // Body Header Section
         Object[] args = new Object[2];
         args[0] = request.getContextPath();
         args[1] = sm.getString("htmlManagerServlet.title");
         writer.print(MessageFormat.format(Constants.BODY_HEADER_SECTION, args));

         // Message Section
         args = new Object[3];
         args[0] = sm.getString("htmlManagerServlet.messageLabel");
         if (message == null || message.length() == 0) {
             args[1] = "OK";
         } else {
             args[1] = RequestUtil.filter(message);
         }
         writer.print(MessageFormat.format(Constants.MESSAGE_SECTION, args));

         // Manager Section
         args = new Object[9];
         args[0] = sm.getString("htmlManagerServlet.manager");
         args[1] = response.encodeURL(request.getContextPath() + "/html/list");
         args[2] = sm.getString("htmlManagerServlet.list");
         args[3] = response.encodeURL(request.getContextPath() + "/"
                 + sm.getString("htmlManagerServlet.helpHtmlManagerFile"));
         args[4] = sm.getString("htmlManagerServlet.helpHtmlManager");
         args[5] = response.encodeURL(request.getContextPath() + "/"
                 + sm.getString("htmlManagerServlet.helpManagerFile"));
         args[6] = sm.getString("htmlManagerServlet.helpManager");
         args[7] = response.encodeURL(request.getContextPath() + "/status");
         args[8] = sm.getString("statusServlet.title");
         writer.print(MessageFormat.format(Constants.MANAGER_SECTION, args));

         // Apps Header Section
         args = new Object[6];
         args[0] = sm.getString("htmlManagerServlet.appsTitle");
         args[1] = sm.getString("htmlManagerServlet.appsPath");
         args[2] = sm.getString("htmlManagerServlet.appsName");
         args[3] = sm.getString("htmlManagerServlet.appsAvailable");
         args[4] = sm.getString("htmlManagerServlet.appsSessions");
         args[5] = sm.getString("htmlManagerServlet.appsTasks");
         writer.print(MessageFormat.format(APPS_HEADER_SECTION, args));

         // Apps Row Section
         // Create sorted map of deployed applications context paths.

         // host就当是当前的tomcat吧，那么contexts就此tomcat下的所有应用
         Container children[] = host.findChildren();
         String contextPaths[] = new String[children.length];
         // 循环每个应用
         for (int i = 0; i < children.length; i++)
             // 应用名称
             contextPaths[i] = children[i].getName();

         TreeMap sortedContextPathsMap = new TreeMap();

         for (int i = 0; i < contextPaths.length; i++) {
             // 应用部署路径
             String displayPath = contextPaths[i];
             sortedContextPathsMap.put(displayPath, contextPaths[i]);
         }

         String appsStart = sm.getString("htmlManagerServlet.appsStart");
         String appsStop = sm.getString("htmlManagerServlet.appsStop");
         String appsReload = sm.getString("htmlManagerServlet.appsReload");
         String appsUndeploy = sm.getString("htmlManagerServlet.appsUndeploy");
         String appsExpire = sm.getString("htmlManagerServlet.appsExpire");

         Iterator iterator = sortedContextPathsMap.entrySet().iterator();
         boolean isHighlighted = true;
         boolean isDeployed = true;
         String highlightColor = null;

         while (iterator.hasNext()) {
             // Bugzilla 34818, alternating row colors
             isHighlighted = !isHighlighted;
             if (isHighlighted) {
                 highlightColor = "#C3F3C3";
             } else {
                 highlightColor = "#FFFFFF";
             }

             Map.Entry entry = (Map.Entry) iterator.next();
             String displayPath = (String) entry.getKey();
             String contextPath = (String) entry.getValue();
             Context context = (Context) host.findChild(contextPath);
             if (displayPath.equals("")) {
                 displayPath = "/";
             }

             if (context != null) {
                 try {
                     isDeployed = isDeployed(contextPath);
                 } catch (Exception e) {
                     // Assume false on failure for safety
                     isDeployed = false;
                 }

                 args = new Object[7];
                 args[0] = URL_ENCODER.encode(contextPath + "/");
                 args[1] = RequestUtil.filter(displayPath);
                 if (context.getDisplayName() == null) {
                     args[2] = "&nbsp;";
                 } else {
                     args[2] = RequestUtil.filter(context.getDisplayName());
                 }
                 managerServlet
                 // 应用是否已启动
                 args[3] = new Boolean(context.getAvailable());
                 args[4] = response.encodeURL(request.getContextPath()
                         + "/html/sessions?path="
                         + URL_ENCODER.encode(displayPath));
                 if (context.getManager() != null) {
                     args[5] = new Integer(context.getManager()
                             .getActiveSessions());
                 } else {
                     args[5] = new Integer(0);
                 }

                 args[6] = highlightColor;
                 //打印出一行关于此应用的信息，应用的URL，当前状态，session数等，具体见上图
                 writer.print(MessageFormat.format(APPS_ROW_DETAILS_SECTION,
                         args));

                 args = new Object[14];
                 args[0] = response
                         .encodeURL(request.getContextPath()
                                 + "/html/start?path="
                                 + URL_ENCODER.encode(displayPath));
                 args[1] = appsStart;
                 args[2] = response.encodeURL(request.getContextPath()
                         + "/html/stop?path=" + URL_ENCODER.encode(displayPath));
                 args[3] = appsStop;
                 args[4] = response.encodeURL(request.getContextPath()
                         + "/html/reload?path="
                         + URL_ENCODER.encode(displayPath));
                 args[5] = appsReload;
                 args[6] = response.encodeURL(request.getContextPath()
                         + "/html/undeploy?path="
                         + URL_ENCODER.encode(displayPath));
                 args[7] = appsUndeploy;

                 args[8] = response.encodeURL(request.getContextPath()
                         + "/html/expire?path="
                         + URL_ENCODER.encode(displayPath));
                 args[9] = appsExpire;
                 args[10] = sm.getString("htmlManagerServlet.expire.explain");
                 Manager manager = context.getManager();
                 if (manager == null) {
                     args[11] = sm.getString("htmlManagerServlet.noManager");
                 } else {
                     args[11] = new Integer(context.getManager()
                             .getMaxInactiveInterval() / 60);
                 }
                 args[12] = sm.getString("htmlManagerServlet.expire.unit");

                 args[13] = highlightColor;

                 if (context.getPath().equals(this.context.getPath())) {
                     writer.print(MessageFormat.format(
                             MANAGER_APP_ROW_BUTTON_SECTION, args));
                 } else if (context.getAvailable() && isDeployed) {
                     writer.print(MessageFormat.format(
                             STARTED_DEPLOYED_APPS_ROW_BUTTON_SECTION, args));
                 } else if (context.getAvailable() && !isDeployed) {
                     writer.print(MessageFormat.format(
                             STARTED_NONDEPLOYED_APPS_ROW_BUTTON_SECTION, args));
                 } else if (!context.getAvailable() && isDeployed) {
                     writer.print(MessageFormat.format(
                             STOPPED_DEPLOYED_APPS_ROW_BUTTON_SECTION, args));
                 } else {
                     writer.print(MessageFormat.format(
                             STOPPED_NONDEPLOYED_APPS_ROW_BUTTON_SECTION, args));
                 }

             }
         }

         // Deploy Section
         args = new Object[7];
         args[0] = sm.getString("htmlManagerServlet.deployTitle");
         args[1] = sm.getString("htmlManagerServlet.deployServer");
         args[2] = response.encodeURL(request.getContextPath() + "/html/deploy");
         args[3] = sm.getString("htmlManagerServlet.deployPath");
         args[4] = sm.getString("htmlManagerServlet.deployConfig");
         args[5] = sm.getString("htmlManagerServlet.deployWar");
         args[6] = sm.getString("htmlManagerServlet.deployButton");
         writer.print(MessageFormat.format(DEPLOY_SECTION, args));

         args = new Object[4];
         args[0] = sm.getString("htmlManagerServlet.deployUpload");
         args[1] = response.encodeURL(request.getContextPath() + "/html/upload");
         args[2] = sm.getString("htmlManagerServlet.deployUploadFile");
         args[3] = sm.getString("htmlManagerServlet.deployButton");
         writer.print(MessageFormat.format(UPLOAD_SECTION, args));

         // Diagnostics section
         args = new Object[5];
         args[0] = sm.getString("htmlManagerServlet.diagnosticsTitle");
         args[1] = sm.getString("htmlManagerServlet.diagnosticsLeak");
         args[2] = response.encodeURL(request.getContextPath()
                 + "/html/findleaks");
         args[3] = sm.getString("htmlManagerServlet.diagnosticsLeakWarning");
         args[4] = sm.getString("htmlManagerServlet.diagnosticsLeakButton");
         writer.print(MessageFormat.format(DIAGNOSTICS_SECTION, args));

         // Server Header Section
         args = new Object[7];
         args[0] = sm.getString("htmlManagerServlet.serverTitle");
         args[1] = sm.getString("htmlManagerServlet.serverVersion");
         args[2] = sm.getString("htmlManagerServlet.serverJVMVersion");
         args[3] = sm.getString("htmlManagerServlet.serverJVMVendor");
         args[4] = sm.getString("htmlManagerServlet.serverOSName");
         args[5] = sm.getString("htmlManagerServlet.serverOSVersion");
         args[6] = sm.getString("htmlManagerServlet.serverOSArch");
         writer.print(MessageFormat
                 .format(Constants.SERVER_HEADER_SECTION, args));

         // Server Row Section
         args = new Object[6];
         args[0] = ServerInfo.getServerInfo();
         args[1] = System.getProperty("java.runtime.version");
         args[2] = System.getProperty("java.vm.vendor");
         args[3] = System.getProperty("os.name");
         args[4] = System.getProperty("os.version");
         args[5] = System.getProperty("os.arch");
         writer.print(MessageFormat.format(Constants.SERVER_ROW_SECTION, args));

         // HTML Tail Section
         writer.print(Constants.HTML_TAIL_SECTION);

         // Finish up the response
         writer.flush();
         writer.close();
     }

　　注意：context.getManager().findSessions()可以取得所有session，但这是个org.apache.catalina.Session[]数组，不是HttpSession[]数组，但这个Session接口里面有个getSession方法，返回结果正是HttpSession类型，没错，就是循环这个数组并调用其getSession方法就可以取得所有在线用户了

上面的Session[]数组是从context对象里面来的，而context是从host对象来的，host是个初始值为NULL的成员变量，是什么时候赋上值的？是在init方法执行前，setWrapper方法执行时赋的值，请看setWrapper方法代码

public class HostManagerServlet
extends HttpServlet implements ContainerServlet

   /**
      * Set the Wrapper with which we are associated.
      *
      * @param wrapper The new wrapper
      */
     public void setWrapper(Wrapper wrapper) {

         //这里所有需要的对象都有了，其实下面我们需要拿到wrapper就够了
         this.wrapper = wrapper;
         if (wrapper == null) {
             context = null;
             host = null;
             engine = null;
         } else {
             context = (Context) wrapper.getParent();
             host = (Host) context.getParent();
             engine = (Engine) host.getParent();
         }

         // Retrieve the MBean server
         mBeanServer = Registry.getRegistry(null, null).getMBeanServer();

     }

　　setWrapper会在初始化时被调用，怎么实现的，首先看web.xml中对此servlet的配置，没什么特别，我们可以发散一下思维，struts2里面action如何能自动注入request对象？Spring如何让service监听事件？答案是一样的，那就是让你的类实现某个接口，你要的东西就给你了，对的，这里也一样，此servlet实现了ContainerServlet接口，初始的时候setWrapper方法才会被调用。

　　是JAVA新手的看这里，我提出上面这些问题，不是想卖什么关子，只是想启发JAVA初学者们，当某天你们做设计时，可以参考这种方法，一句概括就是：只要你实现我的接口，我就可以让你做某事，而不需要任何额外的配置。当然这种设计的缺点就是入侵、偶合。举个简单的应用场景：每天晚上，我用一个定时器通过Spring搜索所有实现了“GarbageCleaner”接口的service bean，并调用其clean方法清理对应模块的垃圾数据，那么任何模块的service只要实现了此接口，就会被调用。

回到正题，我也自已写个servlet并且实ContainerServlet接口吧，使用静态方法取得所有的session，具体代码如下：

 package manager.session.http.servlet;

 import java.io.IOException;
 import java.util.LinkedHashMap;
 import java.util.Map;

 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 import javax.servlet.http.HttpSession;

 import org.apache.catalina.ContainerServlet;
 import org.apache.catalina.Context;
 import org.apache.catalina.Session;
 import org.apache.catalina.Wrapper;

 public class TomcatWrapperServlet extends HttpServlet implements
         ContainerServlet {
     private static final long serialVersionUID = 1L;

     // 弄个静态变量，初始化后就记下来，以备随时使用
     private static Wrapper wrapper = null;

     public Wrapper getWrapper() {
         return wrapper;
     }

     public void setWrapper(Wrapper wrapper) {
         TomcatWrapperServlet.wrapper = wrapper;
     }

     // doGet不做任何事情，只需要接收第一次请求，触发初始动作就完成它的使命了
     @Override
     protected void doGet(HttpServletRequest req, HttpServletResponse resp)
             throws ServletException, IOException {
         resp.getWriter().println("Hello world!");
         resp.getWriter().flush();
         resp.getWriter().close();
     }

     // 初始化后可通过此静态方法取得所有session
     public static Map<String, HttpSession> fillSessions() {
         if (wrapper == null) {// 没有初始化
             throw new RuntimeException(
                     "本servlet未被初始化，您必须先通过URL访问本servlet后，才可以调用这个方法");
         }
         Map<String, HttpSession> sessions = new LinkedHashMap<String, HttpSession>();

         // 取得本应用
         Context context = (Context) wrapper.getParent();
         // 取得Session[]数组
         Session[] temps = context.getManager().findSessions();
         if (temps != null && temps.length > 0) {
             for (int j = 0; j < temps.length; j++) {
                 // Map<sessionId,session>
                 sessions.put(temps[j].getSession().getId(), temps[j]
                         .getSession());
             }
         }
         return sessions;
     }

 }

　

　在web.xml配置一下，然后启动应用，访问之，结果出现异常，是一个安全异常：TomcatWrapperServlet is privileged and cannot be loaded by this web application(想想如下)，说我的类是个特权类，不能被普通的web应用加载，为何manager这个应用又可以呢？把manager/META-INF/context.xml复制到我的应用，再加载，再访问，一切搞定，此文件内容只有一句

　　Xml代码

<Context antiResourceLocking="false" privileged="true" />  

---

HTTP Status 500 - Error allocating a servlet instance

---

type Exception report

message Error allocating a servlet instance

description The server encountered an internal error that prevented it from fulfilling this request.

exception

javax.servlet.ServletException: Error allocating a servlet instance
	org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
	org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
	org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:879)
	org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:617)
	org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1760)
	java.lang.Thread.run(Thread.java:722)

root cause

java.lang.SecurityException: Servlet of class manager.session.http.servlet.TomcatWrapperServlet is privileged and cannot be loaded by this web application
	org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
	org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
	org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:879)
	org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:617)
	org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1760)
	java.lang.Thread.run(Thread.java:722)

note The full stack trace of the root cause is available in the Apache Tomcat/6.0.37 logs.

---

Apache Tomcat/6.0.37

来自:http://www.jspspace.com/ResearchTopics/Art-1757-17.html